1. Field of the Invention
This invention relates to a method of producing pigmented powder coating compositions that have improved appearance. More particularly, it relates to an improved method of melt-blending the constituents of the powder coating.
2. Description of the Prior Art
Powder coating compositions, both with and without pigment, are well known and have been prepared by various methods. All methods have as their objective, of course, to obtain an intimate mixture, in dry, particulate form, of the film-forming materials and pigments that constitute the powder coating. Melt-blending or melt-extrusion has often been used in some stage of many of these methods.
One such method, as shown in U.S. Pat. No. 3,790,513 to Victorius, involves transforming what is essentially a solvent-based coating composition (polymeric resins, crosslinkers, pigment, various additives, all of which are in an organic solvent) into a powder by charging the composition into a vacuum extruder. With heat and vacuum applied, the solvent is removed while the non-volatiles mix and then melt. A 100% solids extrudate is produced which is then ground into a powder. Although powder coatings with good appearance can be produced by this method, the large amounts of solvent which must be removed makes the extruder operation troublesome and expensive, and can pose solvent emission problems.
Another method, shown in U.S. Pat. No. 4,122,060 to Yallourakis, involves charging the previously-blended dry constituents into a conventional melt extruder and then reducing the extrudate to a powder. The dry mixing of the pigment with the other constituents and the additional blending that takes place in the melt-extruder are often insufficient, however, to disperse the pigment uniformly throughout the powder coating. The non-uniform flow of the powder, during application, that can result in poor gloss and distinctness-of-image in the final coating is a disadvantage to the use of this method, especially when the coating is for an automotive topcoat.
Therefore, there remains a need for a melt-blending method of producing pigmented powder coating compositions that disperses pigment uniformly throughout the composition without emitting unduly large amounts of solvent.